Basic understanding of LiPo, Li-Ion and LiFePo4 battery, its use and care

Battery cells


 
Rechargeable Li-ion, Li-Poly and Lithium Iron Phosphate (LiFePO4) batteries differ from the standard Lithium batteries in that the latter are not rechargeable. The lithium-ion batteries Li-Ion usually have a nominal voltage of 3.6V or 3.7V., The LiFePO4  have a nominal voltage of about 3.2V or 3.3V and the lithium-polymer Li-Po batteries have a nominal voltage of 3.6V per cell.
With Li-ion and LiPo batteries the recommended per-cell safety zone is usually between 3V (fully discharged) and 4.2V (fully charged), although they can normally discharge down to about 2.8V without any problems. Dischargingbelow that level may cause irreversible/irreparable damage. Therefore, these batteries often feature a built-in safety mechanisms, preventing over-discharging.  Conversely, overcharging can  alsobe dangerous.
The Li-Po batteries have lower number of recharging cycles than LiFePo4 (1000@0.2C rate, IEC Standard). The projected/estimated life of a Lithium-Ion battery is approximately 3 years from production.
 
 
The LiFePO4  batteries exhibit slightly different properties: They have a bit lower operating voltage of about 3.2V – 3.3V, the minimum discharge voltage is 2.8V and the maximum charged voltage is 3.6V. The LiFePO4 is a kind of Li-Ion rechargeable battery intended for high power applications, such as EV cars , eBikes, electric bike, Power Tools and RC hobby. 
 The LiFePO4 batteries have more constant discharge voltage and are considered to offer better safety than other Lithium-based batteries. Other advantages of the Lithium-based rechargeable batteries include the ability of a much faster recharge and higher discharge rates than other chemistries mentioned and usually higher number of recharge cycles (>2000@0.2C rate, IEC Standard) , meaning longer life when not fully discharged, but its energy density is lower than normal Li-Ion cell (Li-Co)
LiFePO4 life expectancy is approximately 5-7 years.
 
 
 
The S’s:
 
A single LiPo cell has a nominal voltage of 3.6 volts. Since battery packs come in different sizes and voltages, it is more convenient to denote the voltages by saying 10S rather than 36 volts. To find out the nominal voltage of any pack, take the number before the S and multiply it by 3.6. This will render the voltage for the pack. So, a 13S pack will have a nominal voltage of 46.8 volts. 
A single LiFePO4 cell has a nominal voltage of 3.3 volts. Since battery come in all sizes, it is more convenient to denote the voltages by saying 12S rather than 36 volts. To find out the nominal voltage of any pack, take the number before the S and multiply it by 3.3. This will give you the voltage for the pack. So a 16S pack will have a nominal voltage of 52.8 volts
 
  
 
 
LiPo / Li-Ion
 
 
LiFePO4
 
 
Nominal
Voltage
No. of cell
S
Voltage
 S x 3,6V
Charging
Voltage
 S x 4,2V
No. of cell
S
Voltage
 S x 3,3V
Charging
Voltage
 S x 3,6V
12 V
3
4
10,8
14,4
12,6
16,8
4
13,2
14,4
24V
7
25,2
29,4
8
26,4
28,8
36V
10
36,0
42,0
12
39,6
43,2
48V
13
46,8
54,6
16
52,8
57,6
60V
16
57,6
67,2
20
66
72,0
72V
19
68,4
79,8
24
79,2
86,4
 
 
 
 
The P’s:
 
To increase the capacity of the battery pack, additional LiPo cells can be added in parallel to keep the voltage constant while doubling the capacity. Let us take an example of a 10S LiPo with a total capacity of 2000mAh. Now, to get 4000mAh, we have to take another 10S/2000mAh pack  and wire it parallel to the first pack. This would then be denoted by 10S2P, where the P indicates how many 10S packs there are in that battery.
 
 
Ah:
 
The capacity of the battery pack is stated as a rate rather than a quantity. 10Ah means that it will take a 10A load an hour to drain that pack. So if we applied a 20A load to the pack, it would be drained in  half an hour, disregarding internal losses and such. To make a pack last longer, one may want one with a large Ah number. One could also increase the available capacity by taking two identical LiPo and wire the leads in parallel, positive to positive and negative to negative, before connecting it to their eBike or electric bike. That would double the capacity while maintaining the voltage. If we wired the leads in series, one positive lead of battery 1 to one negative lead of battery 2, we would end up with twice the voltage at the same capacity. This is useful when we only have 5S batteries at hand and  a 10S battery is needed. When combining batteries, the participating batteries must all be identical in voltage and capacity.
 
 
C ratings:
 
The first set of C ratings on a LiPo tells us how fast the battery can be discharged. 20C/30C translates to (maximum constant discharge rate) / (Burst discharge rate). Burst can last for up to about 10 seconds. To figure out how many amps this rate is, you need to know the capacity of the battery pack. If the pack says that it has 10Ah, take that number before the Ah  to get the number of amps. In this case, it would be 10. That is your rating of 1C for that pack. Therefore, if the pack says it can safely continuously discharge at 20C, then that means it can supply a constant 100 amps of current. The burst rate would then be 200 amps for 10 seconds. The smaller C rating following the first pair is the charge rate. LiPos must be charged at a much slower rate than its discharge rate. Most packs are good with up to 2C charge rate (for this example, it would be 20 amps).
 
 
Changing - Discharging rates :
 
To charge a LiPo pack, it is highly encouraged to use a charger that supports individual cell balancing or a charger and a battery pack with a BMS. This way, all the cells come off the charger at equal voltages so they all charge equally. When using that charger, you will notice that the charging current drops off as the pack nears its maximum charge, 4.2 volts  for one Li-Po cell and 3.6V volts  for one LiFePO4cell. This is done so the charger does not overcharge the cells which will cause a fire.
 Determined in the same fashion as the C ratings for discharge, the C rating for charge tells you at what amperage you can safely charge your battery.  This information is generally listed on the back of the battery with all the safety information.  For a 10 Ah battery, 2C means that it can be charged at 20A (2*10A).
 
 
Proper Charging:
 
It’s important to use a LiPo compatible charger for LiPos and LiFePO4 compatible charger for  LiFePO4 batteries. They charge using a system called CC/CV charging. It stands for Constant Current / Constant Voltage. Basically, the charger will keep the current, or charge rate, constant until the battery reaches its peak voltage (4.2v/ 3,6V per cell in a battery pack). Then it will maintain that voltage, while reducing the current. On the other hand, NiMH and NiCd batteries charge best using a pulse charging method. Charging a LiPo / LiFePO4 battery in this way can have damaging effects, so it is important to use a LiPo / LiFePO4 compatible charger as appropriate
 
 
Internal Resistance ( IR):
 
Internal Resistance is a measure of the difficulty a battery has delivering its energy to a motor and speed controller. The higher the number, the harder it is for the energy to reach its destination. The energy that doesn’t "go all the way" is lost as heat. So the internal resistance is a kind of a measure of the efficiency of the battery. However, there is a correlation between the C-Rating of a battery and the internal resistance of that battery. In general, batteries with a higher C-Rating also have a low internal resistance.
 
As a general rule: 
·          per cell rating  between 0-6 mΩ is as good as new
·          Between 7 and 12 mΩ is reasonable
·         12 to 20 mΩ is where the signs of lower capacity start to be observed
·          and beyond 20mΩ per cell, one may want to start thinking about retiring the battery pack
 
 
BMS:
 
A battery management system (BMS) is an electronic regulator that monitors and controls the charging and discharging of rechargeable batteries. The BMS comes included in battery pack.
Battery management systems may be as simple as electronics to measure voltage and stop charging when the desired voltage is reached. At that point, they might shut down the power flow; in the event of irregular or dangerous conditions they might issue an alarm. A more complex BMS monitors many factors that affect battery life and performance as well as ensuring safe operation. They may monitor one-cell or multi-cell battery systems. Multi-cell systems may monitor and control conditions of individual cells. Some systems connect to computers for advanced monitoring, logging and more. A more complex BMS also monitors the temperature of a battery pack and can “cut-off”  when the battery temperature is too high while charging or discharging.
 
 
 
Care:
·         A LiPo / Li-Ion cell should NEVER be discharged below 3.0V
·         A LiFePO4 cell should NEVER be discharged below 2.8V
·         Use motor controller with appropriate cut-off Voltage for the battery pack used
·         Charge the battery pack fully after evey use
 
Storage:
·         Always store the batteries fully charged.
·         When not using the cell pack for an extended period of time, remove it from the eBike
          and store in a place with low humidity and low temperature
·         Inspect and Recharge the battery every few months
 
 
Safety:
There are tons of safety precautions related to the use, storage, and disposal of Lithium Polymer batteries.
Some safety precautions:
Each one must be taken seriously since these high-power batteries will pack a punch when not used correctly. There are records of many things -from cars to entire houses - that have burnt down due to the misuse of LiPos. Never leave charging batteries unattended. Never overcharge past 3.6 or 4.2 volts per cell. Over discharging will kill the pack. Drawing too much current from the pack can cause it to puff up and catch fire.
 
 
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Objavljeno: 17. oktober 2014
Zadnja sprememba: 20. november 2014 ob 15:50
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